1. Field of the Invention
The present invention relates to an improvement in magnetic bearing/suspension systems for the near-frictionless support of rotating elements, such as flywheels, electric motors and generators and the like. More specifically, the invention is directed to a special passive bearing element employed in a dynamically stable, passive, totally magnetically energized bearing/suspension system that does not require electrically activated servo controlled systems to attain a stable equilibrium at operating speed.
2. Description of Related Art
Motor and generator armatures, flywheel rotors, and other rotatable components have conventionally been supported and constrained against radially and axially directed forces by mechanical bearings, such as journal bearings, ball bearings, and roller bearings. Such bearings necessarily involve mechanical contact between the rotating element and the bearing components, leading to problems of friction and wear that are well known. Even non-contacting bearings, such as air bearings, involve frictional losses that can be appreciable, and are sensitive to the presence of dust particles. In addition, mechanical bearings, and especially air bearings, are poorly adapted for use in a vacuum environment.
The use of magnetic forces to provide a non-contacting, low friction equivalent of the mechanical bearing is a concept that provides an attractive alternative, one which is now being exploited commercially for a variety of applications. All presently available commercial magnetic bearing/suspension elements are subject to limitations, arising from a fundamental physics issue, that increase their cost and complexity. These limitations make the conventional magnetic bearing elements unsuitable for a wide variety of uses where complexity-related issues, the issue of power requirements, and the requirement for high reliability are paramount.
The physics issue referred to is known by the name of Earnshaw's Theorem. According to Earnshaw's Theorem (when it is applied to magnetic systems), any magnetic suspension element, such as a magnetic bearing that utilizes static magnetic forces between a stationary and a rotating component, cannot exist stably in a state of equilibrium against external forces, e.g. gravity. In other words if such a bearing element is designed to be stable against radially directed displacements, it will be unstable against axially directed displacements, and vice versa. The assumptions implicit in the derivation of Earnshaw's Theorem are that the magnetic fields are static in nature (i. e. that they arise from either fixed currents or objects of fixed magnetization) and that diamagnetic bodies are excluded.
The almost universal response to the restriction imposed by Earnshaw's Theorem has been the following: Magnetic bearing elements are designed to be stable along at least one axis, for example, their axis of symmetry, and then external stabilizing means are used to insure stability along the remaining axes. The "means" referred to could either be mechanical, i. e. ball bearings or the like, or, more commonly, electromagnetic. In the latter approach magnet coils are employed to provide stabilizing forces through electronic servo amplifiers and position sensors that detect the incipiently unstable motion of the rotating element and restore it to its (otherwise unstable) position of force equilibrium.
Less common than the servo-controlled magnetic bearings just described are magnetic bearings that use superconductors to provide a repelling force acting against a permanent magnet element in such a way as to stably levitate that magnet. These bearing types utilize the flux-excluding property of superconductors to attain a stable state, achieved by properly shaping the superconductor and the magnet so as to provide restoring forces for displacements in any direction from the position of force equilibrium. Needless to say, magnetic bearings that employ superconductors are subject to the limitations imposed by the need to maintain the superconductor at cryogenic temperatures, as well as limitations on the magnitude of the forces that they can exert, as determined by the characteristics of the superconductor employed to provide that force.
The magnetic bearing approaches that have been described represent the presently utilized means for creating a stable situation in the face of the limitations imposed by Earnshaw's Theorem. The approach followed by the first one of these (i.e., the one not using superconducting materials) is to overcome these limitations by introducing other force-producing elements, either mechanical, or electromagnetic in nature, that restore equilibrium. The latter, the servo-controlled magnetic bearing, is usually designated as an "active" magnetic bearing, referring to the active involvement of electronic feedback circuitry in maintaining stability.
Recently, U.S. Pat. No. 5,495,221, issued to Post (herein referred to as "Post '221", has described what can be called a "passive" magnetic bearing system. That is, a combination of stationary and rotating elements that together achieve a stable state against perturbing forces without the need for either mechanical, diamagnetic, or electronically controlled servo systems.
Such a system differs fundamentally from previous prior art in that it provides a magnetic bearing system (as opposed to a magnetic bearing element) that can support a rotating object, and that achieves a dynamically stable state, even though any one of its elements, taken alone, would be incapable of stable static levitation. The system described in Post '221 results in reduction in complexity, together with concomitant increases in reliability, reductions in cost, and virtual elimination of power losses that it permits, relative to systems using servo-controlled magnetic bearings.
However, a need still exists to improve such a system. The Post '221 system employs axially symmetric passive levitating elements energized by permanent magnets, and further employs special stabilizer elements to overcome the limitations of Earnshaw's theorem. Nevertheless, suppression of the effects of unbalance or inhibition of the onset of whirl-type rotor-dynamic instabilities driven by displacement-dependent drag forces are achieved in the Post '221 system, if at all, by using vibration dampers that are independently located and separate components from the levitating and/or stabilizer elements.